Question
QUESTION: We have a terrible, disgusting infestation of canker worms, front and back yard, in our 30 oak trees. Will the trees die as a result of this and if not what can we do to treat this and prevent this next year? We are in Winnipeg, Manitoba, Canada. Thank you.

ANSWER: Since the canker worm have only one generation per year the trees will not die from the defoliation. Hardwood trees defoliated in the spring will put on new leaves later in the year.  About all that could happen is some growth loss. If the trees are defoliated for several straight years than you can see die back of some limbs but rarely death. The foliage can be sprayed with an insecticide called Bt which will kill the caterpillars and greatly reduce the population fro next year. The insects predators will eliminate some of the caterpillars and normally the populations vary greatly from year to year so next year there may be little problems. With 30 trees it will be expensive and a lot of work to spray this many.

Canker worms  produce only one generation per year. Once eggs hatch and larvae mature, the event is over for the year. If the trees are sprayed when the spring leaves emerge the caterpillars can be killed. The destructive period may last from three to five weeks, depending on several factors. An insect growth regulator sprayed on the tree and the debris on the ground will kill the caterpillars. Breaking the life cycle will reduce the number of caterpillars.

Don抰 get the idea that devastating infestations occur every year. They don抰. While some worms develop each year, light infestations usually go unnoticed. Predatory insects and diseases serve to keep populations in check. Predicting future infestations is difficult, if not impossible.

The two species of worms mentioned here are incapable of causing physical harm to people. This doesn抰 rule out the possibility that people may discover worms in their hair or on clothing and injure themselves, however.


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QUESTION: Thank you. I have been told that it is too late to spray now as the worms are hanging from the trees already and no longer eating leaves. Is this true? How can I tell when the attack is almost over?

Answer
Bt works as the insect is feeding and if they have completed or near the feeding stage it will do no good to spray. The insects will drop to the ground and construct cocoons where they will stay until early next spring. Guessing I would say 7 -14 days more of seeing caterpillars then the tree will start to leaf out again. In a couple of months you will not be able to tell there was a problem. There is only one life cycle per year. The fact that they pupate in the ground means a large % of the insect population will not make it to next year. due to moisture, temperature and other critters that feed on the pupae.

As the moths hatch out very early next spring the females crawl up the tree trunk to mate and lay eggs.  Banding the trunks of susceptible trees with sticky adhesives such Tanglefoot will trap females as they crawl up the trunk to mate and lay eggs. Trapped females may remain attractive to the males which also get stuck in the sticky band. Tanglefoot is very sticky so I would first put a band of Saran wrap on the trunk 9make sure it is tight so they can not crawl under the wrap and smear the Tanglefoot on this band. That way when you want to take the Tanglefoot off the tree later in the spring it will come off easily and leave no sticky. Tanglefoot can be found at most nursery type stores.